The isolation, by our laboratory and others, of the gene for the precursor of the Alzheimer's disease amyloid polypeptide, has made it possible to begin examining at a molecular level the normal biological functions of this protein in the brain. The use of "reverse genetics" has enabled us to obtain evidence that the amyloid precursor is causally involved in the neurodegeneration of Alzheimer's disease, without yet illuminating its normal role in the brain. It is essential to ascertain the normal biological function of the amyloid precursor, so that we can define the sequence of events whereby this widespread constituent of normal nerve cells is transformed into a peptide that is perhaps toxic to these same cells. A growing body of evidence implicates the amyloid protein (APP) with processes involving the development and modulation of neural connections. Thus, we address the hypothesis that the amyloid precursor is a molecule that is intimately tied to neuronal growth and plasticity. The hypothesis that altered expression of APP is specifically related to the development of new synaptic connections in the mammalian CNS will be tested in three characterized paradigms of neuronal plasticity: adaptive growth responses that occur in the hippocampus following entorhinal lesions, the kindling phenomenon, and long term potentiation. To determine whether APP is associated not only with neural plasticity but also with neuronal growth, the expression of different APP RNAs in primary neurons induced to grow processes will be examined. If a correlation exists, attempt to manipulate neurite outgrowth with APP domain-specific antibodies or to modulate it with bacterial fusion proteins expressing portions of APP will be described. Also proposed are studies to block aspects of neurite outgrowth with antisense oligonucleotides directed toward particular APP mRNAs. Additional studies will analyze the ability of nerve growth factor, a neurotrophic factor for basal forebrain cholinergic neurons, to modulate the expression of APP both in vitro and in vivo. This work is of particular interest due to the severe degeneration of the basal forebrain in Alzheimer's disease. Finally, isolation, sequence analysis, and genetic manipulation of the yeast homologue of the APP gene is proposed. Accomplishment of these objectives will, by contributing to our knowledge of the normal neurobiology of APP, aid us in delineating the sequence of events that disrupts the normal functioning of the amyloid protein precursor, and that concludes with its deposition in the pathological plaques of Alzheimer's disease.